1. Field of the Invention
This invention relates to a method of manufacturing a common rail, such as a high-pressure fuel manifold or a block rail used generally in an accumulator fuel injection system for a diesel internal combustion engine.
2. Description of the Related Art
A known common rail is shown, for example, in FIG. 7, and includes a main tubular rail 11 made of a cross-sectionally circular pipe. Plural bosses 11-4 are provided on an axially extending wall of the rail 11 so that the bosses are spaced from one another. A branch hole 11-2 is formed in each of the bosses 11-4, and communicates with a flow passage 11-1 in the main tubular rail 11. Each branch hole 11-2 has a pressure receiving seat surface 11-3 that is opened outward. A branch pipe 12 is fitted in the branch hole. The branch pipe 12 has a joint head 12-2 with a pressure seat surface 12-3 that is engaged with the pressure receiving seat surface 11-3 of the main tubular rail 11. An external tightening screw type nut 13 is fitted around the branch pipe 12 in advance. The nut 13 is screwed into the relative boss 11-4 to fasten and join the branch pipe 12 to the main tubular rail 11 by pressure of the nut 13 exerted on a bent surface of a neck portion of the joint head 12-2. Referring to FIG. 7, reference numeral 12-1 denotes a flow passage in the branch pipe 12, and 14 denotes a tightening sleeve washer.
The common rail shown in FIG. 7 has the branch hole 11-2 formed in the boss 11-4 that is integral with the main tubular rail 11. Thus, a large tensile stress occurs in an inner circumferential portion P of a lower end of the branch hole 11-2 due to an internal pressure of the main tubular rail 11, and an axial force is exerted on the pressure receiving seat surface 11-3 when the joint head 12-2 of the branch pipe 12 is pressed. Consequently, cracks starting from the inner circumferential portion P of the lower end readily occur, and there is the possibility that leakage of fluid occurs.
To solve such problems, the inventor of the present invention previously proposed a common rail capable of reducing a maximum value of the stress occurring in the inner circumferential portion of the lower end of the branch hole, and thereby improving the internal pressure fatigue strength thereof. One version of this previously proposed common rail includes a main tubular rail having therein an axially extending flow passage. At least one boss is formed on an axially extending circumferential wall of the main tubular rail so as to be integral therewith. A branch hole is formed in the boss and has a pressure receiving seat surface communicating with the flow passage and opened outward. A branch pipe is fitted in the branch hole and has a flow passage communicating with the flow passage in the rail. A pressure seat surface is formed on a joint head at an end portion of the branch pipe, and is engaged with the pressure receiving seat surface. A tightening nut is fitted around the branch pipe in advance. The nut then is screwed into the boss to fasten and join the branch pipe to the main tubular pipe by a pressure occurring due to the screwing force and exerted on a bent surface of a neck portion of the joint head.
An alternate proposal for a common rail included a main tubular rail having therein an axially extending flow passage. At least one branch hole is provided in an axially extending circumferential wall of the main tubular rail. A branch pipe is joined to a circumferential surface portion of the branch hole and has a flow passage communicating with the flow passage of the rail. An outwardly opened pressure receiving surface of the branch hole is engaged with the pressure seat surface on the joint head at the end portion of the branch pipe. A separately formed metal joint is fixed to the main tubular rail. A tightening nut is fitted around the branch pipe in advance, and the metal joint and tightening nut are screwed on each other to press a bent surface of a neck portion of the joint head. Thus, the branch pipe and main tubular rail are fastened and joined to each other.
A second alternate proposal for a common rail included a block rail having a flow passage in an axially extending inner portion. At least one joint hollow is provided in an axially extending circumferential wall of the block rail. A branch hole is provided in the joint hollow. The branch hollow communicates with the flow passage and has an outwardly opened pressure receiving seat surface. A branch pipe is inserted in the branch hole, and has therein a flow passage communicating with the flow passage in the block rail. A pressure seat surface is formed on a joint head provided at an end portion of the branch pipe, and is engaged with the pressure receiving seat surface. A tightening nut is fitted around the branch pipe in advance. The joint hollow and tightening nut are screwed on each other to press a bent surface of a neck portion of the joint head. Thus, the branch pipe and block rail are fastened and joined to each other.
In all of these previously proposed common rails, residual compressive stress is made to exist in a circumferential part of the end portion of the branch pipe which is opened into the flow passage in the main tubular rail or block rail. Thus, stress, which occurs in the inner circumferential part P of a lower end of the branch hole due to the internal pressure of the main tubular rail or block rail and an axial force exerted on the pressure receiving seat surface when the joint head of the branch pipe is pressed, is offset by the residual compressive stress. Thus, a maximum value of the tensile stress occurring in the inner circumferential part P of the lower end of the branch hole is lowered.
The methods of generating and leaving residual compressive stress in the circumferential part of the end portion of the branch pipe which is opened into the flow passage in the main tubular rail or block rail in these common rails include: a method of applying a pressing force to the interior of the flow passage in the main tubular rail or block rail from the outside by a pressing system; a system for applying a pressure to the interior of the main tubular rail or block rail; a pipe expansion system for applying a pressure from the interior of the main tubular rail or a block rail in the radial direction thereof; or a pipe expansion system for applying a pressure from the interior of the branch hole in the radial direction thereof. One method for forming a residual compressive stress includes the steps of forming a bottomed lower hole 11-2' in a boss 11-4 integral with a main tubular rail. The method proceeds by applying a pressing force into the lower hole 11-2' from the outside in the axial direction of the boss by an external pressure system using a punch 15. The punch 15 generates residual compressive stress in a circumferential part of the end portion of a branch hole 11-2 which is opened into a flow passage in the main tubular rail as shown, for example, in FIG.8.
However, a fine defect 16, such as a fine crack, occurs in some cases in a circumferential part of a bottom portion of the lower hole or in a circumferential part of a free end portion of the punch 15 by which a pressing force has been applied to the lower hole. The fine crack is due to tensile stress occurring in a bottom portion of the bottomed lower hole 11-2. FIG. 9 shows a method including the step of applying a pressing force from the outside to a bottomed lower hole 21-2' in a circumferential wall of a main tubular rail 21. The pressing force is applied by an external pressure system using a punch 25 to generate residual compressive stress in a circumferential part of the end portion of a branch hole 21-2 which is opened into a flow passage in the main tubular rail. However, a fine defect 26, such as fine crack, occurs in some cases in the circumferential part of the bottom portion of the lower hole or in a circumferential part of a free end portion of the punch 25 by which the pressing force has been applied to the lower hole. The fine crack is due to tensile stress occurring in the bottom portion of the bottomed lower hole 21-2'. The fine defect, described above, causes a decrease in the fatigue strength of the branch hole, and creates a fear of causing the breakage of the common rail. Therefore, it is necessary that the occurrence of the fine defect be prevented.